The surface structure of silica gel

Peri, J. B.; Hensley, Alexander

doi:10.1021/j100854a041

Cited by 246 publications

(87 citation statements)

References 1 publication

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“…However, at 310~ the hydroxyl content amounts barely to 3.9 OH/100 A 2. This value approximates those recorded by some authors for silica gels (Peri and Hensley, 1968). The limiting value of 6.2 OH/100 A 2 area allows the calculation of the limiting amount of diisocyanate actually fixed through chemical bonding.…”

Section: Determining the Content Of "Isolated" Surface Hydroxyls Of Vsupporting

confidence: 86%

The Surface Reactivity of Vermiculite Towards Hexamethylene Diisocyanate (HMDI)

Siffert¹

1976

Clays and clay miner.

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Abstract--The reaction of hexamethylene diisocyanate with the surface of heat-activatecl vermlcunte parallels the reactions with the oxides SiO2, A1203, MgO considered individually, which a number of authors have already described. The fixation ratio is about 5 molecules per 100 A 2 area. Nearly one half of the grafted molecules retain a "free" isocyanate function, which is requisite for a subsequent polyaddition of the processed mineral. The interaction with vermiculite occurs according to three principal processes; (i) formation of a surface urethane, (ii) formation of a surface carbamate, (iii) fixation by hydrogen bonds.

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Section: Determining the Content Of "Isolated" Surface Hydroxyls Of Vsupporting

confidence: 86%

The Surface Reactivity of Vermiculite Towards Hexamethylene Diisocyanate (HMDI)

Siffert¹

1976

Clays and clay miner.

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“…The theoretical values were reported in literature for the concentration of OH groups on the surface of different types of silica [1,23,25,32,38,80,92,159,160,265,285]. These data show that at present b-cristobalite model of De Boer and Vleeskens is accepted as the correct one.…”

Section: Completely Hydroxylated State Of the Silica Surface Structumentioning

confidence: 59%

The surface chemistry of amorphous silica. Zhuravlev model

Zhuravlev

2000

Colloids and Surfaces A: Physicochemical and Engineering Aspect

2,032

142

1,967

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A review article is presented of the research results obtained by the author on the properties of amorphous silica surface. It has been shown that in any description of the surface silica the hydroxylation of the surface is of critical importance. An analysis was made of the processes of dehydration (the removal of physically adsorbed water), dehydroxylation (the removal of silanol groups from the silica surface), and rehydroxylation (the restoration of the hydroxyl covering). For each of these processes a probable mechanism is suggested. The results of experimental and theoretical studies permitted to construct the original model (Zhuravlev model-1 and model-2) for describing the surface chemistry of amorphous silica. The main advantage of this physico-chemical model lies in the possibility to determine the concentration and the distribution of different types of silanol and siloxane groups and to characterize the energetic heterogeneity of the silica surface as a function of the pretreatment temperature of SiO 2 samples. The model makes it possible to determine the kind of the chemisorption of water (rapid, weakly activated or slow, strongly activated) under the restoration of the hydroxyl covering and also to assess of OH groups inside the SiO 2 skeleton. The magnitude of the silanol number, that is, the number of OH groups per unit surface area, h OH , when the surface is hydroxylated to the maximum degree, is considered to be a physico-chemical constant. This constant has a numerical value: h OH,AVER =4.6 (least-squares method) and h OH,AVER = 4.9 OH nm − 2 (arithmetical mean) and is known in literature as the Kiselev-Zhuravlev constant. It has been established that adsorption and other surface properties per unit surface area of silica are identical (except for very fine pores). On the basis of data published in the literature, this model has been found to be useful in solving various applied and theoretical problems in the field of adsorption, catalysis, chromatography, chemical modification, etc. It has been shown that the Brunauer -EmmettTeller (BET) method is the correct method and gives the opportunity to measure the real physical magnitude of the specific surface area, S Kr (by using low temperature adsorption of krypton), for silicas and other oxide dispersed solids.

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“…Peri [42] proposed the existence of geminal groups over the silica's surface for the first time. He concluded that de-hydrated surface structure of cristobalite at 400 ∘ C includes about 95% geminal hydroxyl groups.…”

Section: Silica Surface Structure (Physisorbed Water Silanol Groups mentioning

confidence: 99%

“…It is now agreed that the relative contribution of the geminals on the silica's surface is relatively small to the total number of silanol groups. Morrow [43] observed that a small quantity of silicon atoms on the silica's surface (less than 5% of the total concentration of silanols) treated at 700 ∘ C carried out geminal silanols; while most of the silicon atoms Figure 11: Representation of different silanol groups (isolated, silanediol and Silanetriol silanols), H-bonded silanols (single and silanediol) alongside siloxanes bridges on the surface of colloidal silica [42] carried single hydroxyl groups being either free or vicinal silanols.…”

Section: Silica Surface Structure (Physisorbed Water Silanol Groups mentioning

confidence: 99%

A review of Fischer Tropsch synthesis process, mechanism, surface chemistry and catalyst formulation

Mahmoudi

Doustdar

et al. 2017

Biofuels Engineering

159

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For more than half a century, Fischer-Tropsch synthesis (FTS) of liquid hydrocarbons was a technology of great potential for the indirect liquefaction of solid or gaseous carbon-based energy sources (Coal-To-Liquid (CTL) and Gas-To-Liquid (GTL)) into liquid transportable fuels. In contrast with the past, nowadays transport fuels are mainly produced from crude oil and there is not considerable diversity in their variety. Due to some limitations in the first generation bio-fuels, the Second-Generation Biofuels (SGB)' technology was developed to perform the Biomass-To-Liquid (BTL) process. The BTL is a well-known multi-step process to convert the carbonaceous feedstock (biomass) into liquid fuels via FTS technology. This paper presents a brief history of FTS technology used to convert coal into liquid hydrocarbons; the significance of bioenergy and SGB are discussed as well. The paper covers the characteristics of biomass, which is used as feedstock in the BTL process. Different mechanisms in the FTS process to describe carbon monoxide hydrogenation as well as surface polymerization reaction are discussed widely in this paper. The discussed mechanisms consist of carbide, COinsertion and the hydroxycarbene mechanism. The surface chemistry of silica support is discussed. Silanol functional groups in silicon chemistry are explained extensively. The catalyst formulation in the Fischer Tropsch (F-T) process as well as F-T reaction engineering is discussed. In addition, the most common catalysts are introduced and the current reactor technologies in the F-T indirect liquefaction process are considered. process overview IntroductionThe over-reliance of the world's nations on conventional fossil fuels puts our planet in peril. The continuity of the current situation will result in the rise of a combined average temperature over global land and ocean surfaces by 5 ∘ C in 2100, bringing a rise in sea levels, food and water shortages and an increase in extreme weather events. The global warming, caused by humans, is one of the biggest threats to our future well-being [1]. In addition, oil reserves are limited and these reserves are decreasing dramatically. This reduction alongside the other relevant economic factors affects the world's oil prices. The need to run engines with the new generation of liquid fuels is inevitable. The investigations by the US Energy Information Administration (EIA) published in 2013 expressed a 56 percent increase in the world's energy consumption by the year 2040. Total world energy demand will have risen to 865 EJ (exajoule) by this year. The total world energy consumption was reported as 553 EJ in 2010. The outlook indicates that renewable energy is one of the fastest-growing energy sources in the world; where its usage increases 2.5 percent per year. Despite increasing success in the renewable energies, it is predicted that the fossil fuels will supply al-

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The surface structure of silica gel

Cited by 246 publications

References 1 publication

The Surface Reactivity of Vermiculite Towards Hexamethylene Diisocyanate (HMDI)

The Surface Reactivity of Vermiculite Towards Hexamethylene Diisocyanate (HMDI)

The surface chemistry of amorphous silica. Zhuravlev model

A review of Fischer Tropsch synthesis process, mechanism, surface chemistry and catalyst formulation

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